Trophic ecology of bullet tuna larvae (Auxis rochei) in the Northwestern Mediterranean

Abstract

Bullet tuna (Auxis rochei) is a small tuna of tropical and temperate distribution that performs a reproductive migration from the Atlantic to the Mediterranean Sea. Studies on the trophic ecology of tuna larvae, and particularly on Auxis spp. are very scarce. This constitutes the fist attempt to jointly study the diet and selectivity in larvae of this species. Specimens were collected in the Catalan Sea in June 2005 by means of oblique Bongo hauls (fish larvae) and vertical Calvet tows (microplankton and mesozooplankton). Larvae appeared all along the study area, with main concentrations in the southwestern sector, where sea surface temperature was higher than 23°C. Maximum values of sea surface fluorescence were recorded also in this sector, where lower salinities were also found. Larval size ranged from 2 to 7 mm SL. Larval feeding incidence was high during daylight hours (87%) and low during the night (19%). Prey number ranged from 0 to 13 (mean 5±3). Diet was based on phytoplankton, copepods (eggs, nauplii and postnauplii), cladocerans, apendicularians, tintinnids, gastropoda larvae, equinodermata larvae, invertebrate eggs and fish larvae. Prey diversity was higher in preflexion larvae. Piscivory was only observed in larvae larger than 5 mm. Although an increasing prey size with larval development was observed, no increase in the trophic niche breadth was found. Chesson Selectivity index indicated that small larvae (2-3 mm) selected phytoplankton, copepods eggs and nauplii, and gastropoda larvae. From 3-5 mm copepod nauplii, cladocerans and apendicularians were selected. Larvae ≥5 mm preferred copepod postnauplii, cladocerans and appendicularians